1. Field of Invention
The present invention relates to semiconductor light emitting devices with a photonic band gap material disposed between the device and a luminescent material layer.
2. Description of Related Art
Semiconductor light emitting devices such as light emitting diodes (LEDs) are among the most efficient light sources currently available. Material systems currently of interest in the manufacture of high brightness LEDs capable of operation across the visible spectrum include group III–V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials; and binary, ternary, and quaternary alloys of gallium, aluminum, indium, and phosphorus, also referred to as III-phosphide materials. Often III-nitride devices are epitaxially grown on sapphire, silicon carbide, or III-nitride substrates and III-phosphide devices are epitaxially grown on gallium arsenide or gallium phosphide by metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The devices include an active, light emitting region sandwiched between an n-type region and a p-type region. Electrical contacts are provided on the n- and p-type regions.
The color of light emitted from a semiconductor light emitting device may be altered by placing a wavelength-converting material in the path of the light exiting the chip. The wavelength-converting material may be, for example, a phosphor. Phosphors are luminescent materials that can absorb an excitation energy (usually radiation energy) and store this energy for a short period of time. The stored energy is then emitted in the form of a photon of a different energy than the initial excitation energy. For example, “down-conversion” refers to a situation where the emitted photons have less energy than the excitation photon energy. The photon wavelength increases (since E=hc/λ), shifting the color of the light towards red.
FIG. 1 illustrates a prior art phosphor-converted LED 10 based on gallium nitride (GaN), described in more detail in U.S. Pat. No. 6,155,699 to Miller et al. The LED 10 includes a GaN die 12 that generates blue primary light when activated. The GaN die 12 is positioned on a reflector cup lead frame 14 and is electrically coupled to leads 16 and 18. The leads 16 and 18 provide electrical power to the GaN die 12. The GaN die 12 is covered by a region 20 of phosphorescent material. The type of phosphorescent material utilized to form the region 20 can vary, depending upon the desired color of secondary light that will be generated by the region 20. The GaN die 12 and the phosphorescent region 20 are encapsulated by a lens 22. The lens 22 is typically made of a transparent epoxy.
In operation, electrical current is supplied to the GaN die 12 to activate the GaN die. When activated, the GaN die 12 emits the primary light, i.e., blue light, away from the top surface of the GaN die 12. The emitted primary light is absorbed by the phosphorescent region 20. The phosphorescent region 20 then emits secondary light, i.e., the converted light having a longer peak wavelength, in response to absorption of the primary light. The secondary light is emitted randomly in various directions by the phosphorescent region 20. Some of the secondary light is emitted away from the GaN die 12, propagating through the lens 22 and exiting the LED 10 as output light. The lens 22 directs the output light in a general direction indicated by arrow 24.
However, some of the secondary light is emitted from the phosphorescent region 20 toward the GaN die 12. The amount of secondary light that is emitted toward the GaN die 12 may be as much as half of the generated secondary light. Much of this light is then absorbed by the semiconductor layers and contact metals within GaN die 12. Therefore, the amount of secondary light that eventually escapes the LED 10 as output light is significantly reduced. The maximum external conversion efficiency of typical prior art LEDs, such as the LED 10, has been estimated to be about 60%. The maximum external conversion efficiency is the percentage of output light with respect to the emitted primary light.
U.S. Pat. No. 5,813,752 to Singer et al. describes a phosphor conversion LED that includes a “short wave pass” (SWP) filter to mitigate the above-described cause of inefficiency. The Singer et al. LED is virtually identical to the LED 10, except that the SWP filter is positioned on the upper surface of the GaN die 12, sandwiched between the GaN die 12 and the phosphorescent region 20. The SWP filter transmits the primary light from the GaN die 12, but reflects a portion of the secondary light emitted from the phosphorescent region 20. Therefore, the portion of secondary light that is emitted toward the GaN die 12 from the phosphorescent region 20 is reflected by the SWP filter and transmitted through the lens 22, contributing to the total output light. The reflecting of the secondary light by the SWP filter is designed to increase external conversion efficiency.
A concern with the Singer et al. LED is that the SWP filter does not have well-behaved transmittance and reflectance characteristics to efficiently transmit and reflect the primary light and the secondary light over a range of angles, respectively. Ideally, the SWP filter should operate to transmit all of the primary light from the GaN die 12 to the phosphorescent region 20 and reflect all of the secondary light that is emitted back toward the GaN die 12 from the phosphorescent region 20. The concern stems from the fact that the emitted primary light impinges upon the SWP filter with a large range of incident angles. The incident angle is the angle measured from the propagating direction of the incident light to the normal, a perpendicular line with respect to the surface of the SWP filter. The SWP filter exhibits a strong angular dependency, and the large range of incident angles results in undesirable transmittance and reflectance characteristics. In particular, the SWP filter reflects a significant portion of the primary light generated within the GaN die and emitted toward the SWP filter at high incidence angles back into GaN die 12. Therefore, that portion of the emitted primary light will not be transmitted through the SWP filter to the phosphorescent region 20. The decrease in the amount of primary light transmitted through the SWP filter consequently decreases the amount of primary light that can be converted by the phosphorescent region 20. Thus, the amount of secondary light generated by the phosphorescent region 20 is also reduced. The overall effect is that the amount of output light from the Singer et al. LED has been reduced by the less than optimal rate of transmittance of the SWP filter.
Miller et al. proposes a solution to the problems presented by Singer et al., shown in FIG. 2. The GaN die 12 is covered by an encapsulating layer 28 made of a transparent material. The transparent material may be clear epoxy or glass. The encapsulating layer 28 is a dome-shaped structure that surrounds the die 12. In the preferred embodiment of Miller et al., the dome-shaped structure is a generally hemispheric structure having a radius that equals the distance from A to C. The distance from A to C is equal to or greater than three times the distance from A to B. Adjacent to the encapsulating layer 28 is a distributed Bragg reflector (DBR) mirror 30. Because the DBR mirror 30 is formed over the upper surface of the encapsulating layer 28, the DBR mirror 30 is a dome-shaped shell. A layer 36 of phosphorescent material is located over the DBR mirror 30. The DBR mirror allows much of the primary light to be transmitted through the DBR mirror to the phosphorescent layer. The amount of primary light transmitted through the DBR mirror depends on the transmittance characteristic of the DBR mirror. Moving the DBR mirror some distance away from the GaN die decreases the range of incident angles. The small range of incident angles increases the transmittance of the primary light.
There are several problems with the design proposed by Miller et al. First, though spacing DBR mirror 30 apart from die 12 may decrease the range of incident angles, Miller's DBR 30 still transmits primary light only in a limited range of angles of incidence. Second, Miller's DBR 30 reflects secondary light emitted by Miller's phosphorescent material layer 36 only in a limited range of angles of incidence. Since light is emitted by phosphorescent material layer 36 in random directions, a significant portion of the secondary light may be transmitted by DBR 30 into die 12 and absorbed. Third, the thickness of the layers of Miller's DBR 30 must be precisely controlled in order to achieve the desired reflectance and transmittance characteristics. Since DBR 30 is formed on a curved surface, fabrication of DBR 30 with the thickness control required may be difficult and expensive. Fourth, since Miller's DBR 30 is spaced apart from die 12, the source size of Miller's is undesirably larger than a device without DBR 30, limiting the range of practical applications of Miller's device.